In the related art, as an optical scanning device installed in an electrophotographic image forming apparatus, there has been known an optical scanning device including a rotating polygon mirror that deflects and scans light beams emitted from a light source and a driving motor that drives the rotating polygon mirror (for example, see Patent Literature 1). The rotating polygon mirror is provided in a bottomed box-like casing in which a ceiling side is opened, and the driving motor is fixed to a bottom wall of the casing. The ceiling side of the casing is closed by a lid member. The casing and the lid member are fixed to each other via a fixing mechanism. For an example of the fixing mechanism, there may be a mechanism and the like that cause an engaging claw and an engaging hole to be engaged with each other.
The casing is formed with an inspection hole having an inspection function for performing optical axis adjustment of the light source when the optical scanning device is assembled and a wiring hole having a function for drawing out an electric wiring connected to the driving motor. The inside and outside of the casing communicate with each other via the inspection hole and the wiring hole. The inspection hole is closed by an adhesive tape and the like after completion of assembling the optical scanning device. In a space between the wiring hole and the electric wiring or in a space between the casing and the lid member, a seal member (for example, an elastic member) is provided. In this way, foreign matters (for example, toner, paper powder and the like) outside the casing are prevented from entering into the casing from the space after the casing enters a negative pressure state due to the rotation of the rotating polygon mirror.